In recent years, nonaqueous electrolyte batteries using carbon material, metallic lithium or lithium-alloying material for the negative active material and a lithium transition metal complex oxide represented by LiMO2 (M indicates a transition metal) for the positive active material have been noted as high-energy-density secondary batteries.
A representing example of the lithium transition metal complex oxide is a lithium cobalt complex oxide (lithium cobaltate: LiCoO2). This complex oxide has been already put to practical use as the positive active material of nonaqueous electrolyte secondary batteries.
The lithium transition metal complex oxide containing Ni or Mn as the transition metal has been also studied for utility as the positive active material. For example, such compounds as containing all of those three transition metals Co, Ni, and Mn have been also studied intensively (see, for example, Japanese Patent Registration Nos. 2,561,556 and 3,244,314, and Journal of Powder Sources, 90(2000), pp 176-181).
It is reported that, among those lithium transition metal complex oxides containing Co, Ni and Mn, a compound containing the same percentage composition of Ni and Mn, as represented by the formula LiMnxNixCo(1-2x)O2, shows characteristically high thermal stability even in the charged state (high oxidation state) (Electrochemical and Solid-State Letters, 4(12), A200-A203(2001)).
It is also reported that the aforementioned complex oxide containing substantially the same percentage composition of Ni and Mn has a voltage of around 4 V, comparable to a voltage of LiCoO2, and shows a high capacity and a good charge-discharge efficiency (Japanese Patent Laying-Open No. 2002-42813). Thus, batteries using such a lithium transition metal complex oxide (such as represented by the formula LiaMnbNibCo(1-2b)O2 (0≦a≦1.2 and 0<b≦0.5)) containing Co, Ni and Mn and having a layered structure as the positive electrode material are expected to exhibit high thermal stability even in the charged state and accordingly exhibit markedly improved battery reliability.
The inventors of this application studied performance characteristics of the secondary lithium battery using the above-described lithium transition metal complex oxide as the positive active material. As a result, they have found that when the battery is stored in the charged state at a temperature that is higher than 80° C., which temperature is estimated as the use condition of a portable phone inside an actual car, an increasing gas evolution due probably to a reaction between the positive electrode and the electrolyte solution occurs to expand or swell the battery if having a configuration suitable for insertion in the portable phone or the like. In an exemplary case where a battery casing is made of a thin aluminum alloy or a thin aluminum laminate film, the battery when stored has been found to experience significant swelling and show marked deterioration such as capacity loss.
Batteries often use an outer casing formed of a thin aluminum alloy or a thin aluminum laminate film to reduce their weights. As a solution to suppress swelling of such batteries due to gas evolution during high-temperature storage, a method is proposed wherein γ-butyrolactone is contained as a solvent for an electrolyte solution in the amount of 50-95% by volume (see, for example, Japanese Patent Laying-Open No. 2000-235868). However, in this case, because γ-butyrolactone is susceptible to decomposition at a reducing side (at a negative electrode side), the performance characteristics of the battery have been insufficient in total.
In Japanese Patent Laying-Open No. 2002-203552 and the 43rd Battery Symposium in Japan Meeting Abstracts, pp. 122-123, the use of a complex metal oxide comprised mainly of Li and Ni and having a pH value of 10.0-11.5 as the positive electrode material for a nonaqueous electrolyte secondary battery is proposed to suppress swelling of the battery during high-temperature storage. However, as a result of the detailed study on lithium transition metal complex oxides containing Ni, Mn and Co and having a layered structure, the inventors of this application have found that the use of such lithium transition metal complex oxides, even if kept within the specified pH range, results in significant swelling of the batteries during high-temperature storage in the charged state and thus the failure to obtain a sufficient improvement.